The present invention relates to a heating apparatus utilizing an induction heating, particularly, to a fixing apparatus used in, for example, an electrophotographic copying apparatus or a printer apparatus using a toner as a visualizing agent so as to fix the toner image.
In a fixing apparatus incorporated in a copying apparatus using an electrophotographic process, a developing agent image, i.e., a toner image, formed on a transferred material is melted by heating so as to permit the toner image to be fixed to the transferred material. Various methods of heating the toner, which can be employed in a fixing apparatus, have been put to a practical use including, for example, a method utilizing a radiation heat radiated from a halogen lamp (filament lamp), a flash heating method utilizing a flash lamp as the heat source, an oven heating method for heating the entire fixing portion with a heat source, and a hot plate heating system in which plates having a transferred material sandwiched therebetween are heated, a toner image being formed on said transferred material.
In the method using a halogen lamp as a heat source, it is widely known to the art to use a pair of rollers arranged to be capable of applying a predetermined pressure to the transferred material and the toner held therebetween. In this case, at least one of the rollers is formed hollow and a columnar halogen lamp is arranged within the inner space of the hollow roller. In the method of using a halogen lamp, a nip portion is formed between the hollow roller having the halogen lamp arranged therein and the other roller, i.e., a pressurizing roller, and these two rollers are rotated to permit the transferred material having a toner image formed thereon to be guided into the nip portion, with the result that pressure and heat are applied to the transferred material having a toner image formed thereon.
To be more specific, the transferred material, e.g., a paper sheet, having a toner image formed thereon is guided to the nip portion between the hollow heating roller having a halogen lamp arranged therein and the rotating pressurizing roller so as to melt the toner on the paper sheet and, thus, to fix the toner image to the paper sheet.
In the fixing apparatus using a halogen lamp, however, the light and the infrared rays are radiated from the halogen lamp in the entire circumferential direction of the heating roller so as to heat the entire heating roller. As a result, the heat conversion efficiency is 60 to 70% because of the loss in the conversion of the light into heat and the efficiency of warming the air within the heating roller and transmitting the heat to the heating roller, leading to a large power consumption. In addition, a long warming time is required.
Under the circumstances, an improved fixing apparatus has been proposed in recent years. Specifically, it is proposed that a heat resistant film having a thin metal layer (conductor) movable in tight contact with a heating body is formed into an endless belt or a cylinder, and a material to be heated, which is in tight contact with the heat resistant film, is moved together with the film so as to permit the film to impart the heat energy of the heating body to the material to be heated. Incidentally, the heat resistant film has in general a width conforming with the maximum width of the transferred material. It follows that, in the fixing apparatus using the particular film, it is necessary to control the temperature to be uniform in the longitudinal direction of the heating body. As a result, the uniformity in the manufacturing step and the temperature control with a high accuracy in the operating step are required, leading to an increased manufacturing cost of the apparatus.
Incidentally, when it comes to a high speed copying machine capable of making a large number of copies per unit time, the heating time is shortened in general. As a result, it is necessary to use a heating body having a large heat capacity, leading to an increased power consumption. Also, the total power consumption is rendered unduly large so as to give rise to an inconvenience in terms of the safety standards.
In order to overcome the above-noted problems inherent in the heater fixation and the film fixation, a fixing apparatus using an induction heating is proposed in, for example, Japanese Patent Disclosure (Kokai) No. 9-258586 and Japanese patent Disclosure No. 8-76620.
Specifically, Japanese Patent Disclosure No. 9-258586 discloses a fixing apparatus, in which an electric current is allowed to flow through an induction coil prepared by winding a coil about a core arranged along the rotary shaft of the fixing roller made of a metal so as to generate an induction current in the roller and, thus, to permit the metal roller itself to generate heat.
On the other hand, Japanese Patent Disclosure No. 8-76620 discloses a fixing apparatus comprising a conductive film having a magnetic field generating means housed therein and a pressurizing roller that is in tight contact with the conductive film. In this prior art, heat is generated from the conductive film so as to permit the toner image formed on a transferred material, which is transferred through the clearance between the conductive film and the pressurizing roller, to be fixed to the transferred material.
In the fixing apparatus of the induction heating system described above, it is known to the art that the surface temperature is not necessarily rendered uniform in the circumferential direction of the roller body in conjunction with the winding direction of the exciting coil constituting the magnetic field generating device, though the roller body is heated uniformly in the circumferential direction in the known fixing apparatus using a halogen lamp. In other words, the problem is generated that the detected surface temperature is rendered inaccurate depending on the position at which the temperature detecting mechanism is mounted for detecting the surface temperature on the outer circumferential surface of the roller body.
For example, Japanese Patent Disclosure No. 2000-172100 discloses a fixing apparatus, in which an exciting coil is wound about a bobbin to form a solenoid such that the axis of the roller body is equal to the axis of the coil. In this case, the temperature distribution is rendered uniform in the circumferential direction of the roller body, making it possible to detect the surface temperature accurately regardless of the mounting position of the temperature detecting means for detecting the temperature of the roller body on the outer circumferential surface of the roller body. However, the coil of the solenoid type is small in its magnetic flux generating force and, thus, is low in efficiency. In order to increase the efficiency, it is necessary to the number of turns of the coil.
On the other hand, in order to increase the magnetic flux generating force, i.e., to increase the efficiency of the coil, a transverse system has been put to a practical use, in which the winding direction of the coil is allowed to form a plane extending along the circumferential surface of the roller body in order to prevent the magnetic flux generated in the circumferential direction of the roller body from becoming uniform, and the most portion of the wire material forming the coil is allowed to extend in the longitudinal direction of the coil. In this case, distribution takes place in the surface temperature in the circumferential direction of the roller body, making it necessary to mount the temperature detecting mechanism in a position where the surface temperature of the roller body can be detected accurately.
On the other hand, in order to decrease the power consumption (waiting power) by shortening the warm up time, the heat capacity of the roller body is decreased by decreasing the wall thickness of the roller body. However, if a paper sheet differing in size is utilized as a result of decreasing the heat capacity of the roller body, a new problem is generated that the temperature is rendered nonuniform in the longitudinal direction of the roller body. For example, if a paper sheet of a small size, i.e., a paper sheet having a small width relative to the length of the roller body, is consecutively supplied to the fixing device, the heat is not consumed in the edge portion of the roller body, though a large amount of heat is consumed in the central portion of the roller body, giving rise to a problem that the temperature in the edge portion of the roller body exceeds specified temperature. Incidentally, where the roller body has a large heat capacity, the temperature is rendered uniform in the longitudinal direction of the roller body because of the heat conduction. However, where the wall thickness of the roller body is small, it is difficult to eliminate the temperature difference in the longitudinal direction of the roller body.
Under the circumstances, it is disclosed in, for example, Japanese Patent Disclosure No. 9-244465 that a temperature detecting apparatus is mounted to an edge portion of the roller body. It is disclosed that, where the detected temperature exceeds a predetermined value, a cooling fan mounted to an edge portion of the roller body is operated so as to cool the edge portion of the roller body. However, if the roller body is cooled in spite of the temperature in the central portion of the roller body, with the temperature in the edge portion of the roller body used as the threshold value, the temperature in the central portion of the roller body is also lowered. As a result, an additional heating is required so as to increase the power consumption.
Incidentally, in accordance with the recent miniaturization of the image forming apparatus, the fixing apparatus and the image forming portion are arranged close to each other. As a result, the heat of the fixing apparatus brings about a problem that the temperature in the vicinity of the photosensitive body included in the image forming section is elevated to the upper limit of the temperature at which the photosensitive body can be used.
An object of the present invention is to provide a fixing apparatus of the type that a temperature distribution is generated in the circumferential direction of the roller body, in which the temperature of the roller body can be detected accurately.
Another object of the present invention is to provide a fixing apparatus, in which, even if a transferred material is left unremoved within the apparatus in the case where the temperature of the roller body is elevated to an abnormally high temperature, it is possible to detect the abnormal temperature before the transferred material is caused to flame so as to stop the heating of the roller body.
Another object of the present invention is to provide a fixing apparatus capable of cooling an edge portion of the roller body in the case where the temperature elevation has reached a predetermined temperature at the edge portion of the roller body, which takes place in the case where the size of the transferred material differs. In the fixing apparatus of the present invention, the number of cooling operations is made optimum so as to suppress the temperature gradient in the longitudinal direction of the roller body.
Further, still another object of the present invention is to provide a fixing apparatus capable of suppressing the temperature elevation in the periphery of the photosensitive body included in the image forming section while suppressing the temperature difference in the longitudinal direction of the roller body.
According to a first aspect of the present invention, there is provided a fixing apparatus, comprising:
an endless member having a metal layer made of a conductor and rotatable in an optional direction;
a pressurizing member serving to apply a predetermined pressure to the endless member;
an induction heating apparatus arranged in the vicinity of the endless member so as to heat the endless member; and
a temperature detecting apparatus for detecting the temperature of the endless member, the temperature detecting apparatus being arranged at a point of about 90xc2x0 along the outer circumferential surface of the endless member from the pressurizing point at which the pressurizing member applies pressure to the endless member.
According to a second aspect of the present invention, there is provided a fixing apparatus, comprising:
an endless member having a metal layer made of a conductor and rotatable in an optional direction;
a pressurizing member serving to apply a predetermined pressure to the endless member;
an induction heating apparatus arranged in the vicinity of the endless member so as to heat the endless member;
a first temperature detecting apparatus arranged in that position on the surface of the endless member through which passes the minimum size of a transferred material so as to detect the temperature of the endless member;
a second temperature detecting apparatus arranged outside that region on the surface of the endless member through which passes the maximum size of a transferred material so as to detect the temperature of the endless member; and
cooling means for cooling the edge portion of the endless member in accordance with the difference in temperatures detected by the first temperature detecting apparatus and the second temperature detecting apparatus.
According to a third aspect of the present invention, there is provided an image forming apparatus, comprising:
a photosensitive body holding a latent image;
a developing apparatus for supplying a developing agent onto the latent image formed on the photosensitive body so as to visualize the latent image;
a transfer apparatus for transferring the developing agent image formed on the photosensitive body by the developing apparatus onto a transferred material;
a fixing apparatus having an endless member including a metal layer formed of a conductor and rotatable in an optional direction, a pressurizing member for applying a predetermined pressure to the endless member, and a heating device arranged in the vicinity of the endless member for heating the endless member, the transferred material having the developing agent image transferred thereto by the transfer apparatus and the developing agent image itself being heated and pressurized so as to fix the developing agent image onto the transferred material; and
a cooling apparatus arranged between a first member selected from the endless member and the heating device of the fixing apparatus and a second member selected from the photosensitive body, the developing apparatus and the transfer apparatus, the first member being positioned closest to the second member, for cooling at least one of the edge portion of the endless member and apparatus positioned closest to the second member.